Quasi-static cone penetrometers of various types have heretofore been used extensively to determine the engineering properties of various soils, whether they be clays, sands, or loams. The principal measurement has been the resistance to penetration of the penetrometer into the soil at a constant velocity. The penetrometer has typically been a standard cone-tipped cylindrical shaft with apparatus for measuring tip resistance, such data being recorded against depth. A limitation of such penetrometers heretofore has been that the type of soil being penetrated was not always readily identified and that such identification is requisite for successful interpretation of the tip resistance data.
It has been found that the amount of frictional resistance to penetration of a smooth cylindrical sleeve that is above and connected to the penetrometer cone tip is also useful in understanding the soil characteristics and thus for geotechnical design. This, too, can be measured by the same instrument when it is provided with additional apparatus.
Until recently, a so-called friction ratio, obtained by dividing the measured frictional resistance by the measured tip resistance, has been the principal method by which attempts were made to distinguish different soil types from one another during penetration testing. Such distinction of soil type, however, is possible only between sands and clays, and is not considered to be reliable, even for those widely different grain sizes. For example, research has shown that four sands identical in all aspects except for differing in the important parameter of grain size, yielded essentially identical "friction ratios"; it would therefore not be possible to distinguish between the four sands on the basis of friction ratio.
A device able to generate these two types of data may comprise a cone tip with a tip load cell, a friction sleeve above the tip with a friction load cell, and a common shaft above the tip threaded to the tip load cell and to the friction sleeve load cell. Leads from the load cells pass up through a hollow core of the common shaft to the upper end of the penetrometer and thence through penetrometer rods to the ground surface and to suitable recording apparatus.
Frictional resistances are also required for the design of friction piles. The adhesion of soil to the smooth metal jacket of the friction sleeve, may not necessarily be identical to the adhesion of the soil to a concrete, wood or rough iron pile; the measured friction resistance on the smooth friction sleeve is, however, a very good indicator of what the magnitudes of such adhesion may be.
It has been determined that as a rigid object, such as a penetrometer, is pushed into a soil, acoustic emissions are generated by soil grains sliding and rolling over one another, sliding and rolling over the penetrating object, and being crushed. Little use of such acoustic emissions has heretofore been made, and none, so far as we are aware, in a penetrometer which can transmit such acoustic emissions simultaneously with the measurement of cone tip penetration resistance and friction sleeve resistance. Such acoustical response is, in this invention, detected by an acoustical transducer located within the penetrometer and recorded on magnetic tape as well as being amplified for direct listening during the penetration tests.
We have found that a greatly improved identification in situ of soil types and strata boundaries, can be obtained by simultaneously obtaining and recording all three types of data.
In order to do this in an optimum manner one must solve the problem of preventing the noise generated by soil grains moving over the friction sleeve from interfering with and modifying the acoustical data generated by the grains moving over the conical tip. In other words, one must so isolate the acoustic tip that it does not receive acoustic information from the remainder of the penetrometer and its associated rods.